God had every reason in the world to kill every airbreather on earth. Mankind had decided, not unlike today, that they would rather be their own gods than to worship the true God. God found only one man on all the earth that loved Him, and He saved him and his family from destruction. God had every reason in the world to destroy humanity completely, but because of the faithfulness of one man, he did not. He saved him, his family, and all the animals of the world.
A common refrain today is that God is different from the way he was in the Old Testament. There are enough episodes of His mercy to humankind and His steadfast love even in the face of rejection in the Tanakh that prove otherwise. To be faithful to lovingly preserve His creation for the sake of one man is evidence enough of His mercy.